FIG. 1 illustrates a conventional DC chopper motor drive. A full-wave bridge rectifier 11 receives an AC line voltage and constantly provides a rectified DC voltage to charge one or more capacitors 12. Coupled in parallel with capacitors 12 are motor M and switch 13. Motor M is coupled in series with switch 13, which may be a solid-state device such as a MOSFET or IGBT. The state of switch 13 controls whether current flows through motor M, and the state of switch 13 is in turn controlled by a signal from a motor speed control loop 14. When switch 13 is closed, power flows from capacitors 12 to the armature of DC motor M. The speed of DC motor M is determined largely by the voltage across capacitors 12 and the duty cycle of the signal supplied to switch 13.
The motor speed control loop 14 controls switch 13 with a pulse-width modulated (PWM) signal, which determines the state of switch 13 and the operation of motor M. Typically, the shaft of motor M may be configured to transmit a pulse train whose frequency indicates the speed at which the shaft is rotating. Other suitable approaches may utilize motor speed feedback means such as an analog tachometer or an encoder. The motor speed control loop 14 may function by receiving a desired motor speed, synthesizing a pulse frequency corresponding to that desired motor speed, determining the current motor speed by analyzing the frequency of the pulse train from the motor shaft, and adjusting the PWM signal so that the actual motor speed approaches the desired motor speed.
Typically, the voltage on the capacitors 12 is unregulated, that is, the voltage is not externally controlled to be within a certain range. Thus, in FIG. 1, the voltage on capacitors 12 is determined largely by the magnitude of the AC line voltage. The motor speed control loop 14 can adjust the speed of the motor M only by adjusting the PWM signal. Should the capacitor voltage fluctuate for some reason, such as load variations, the motor speed control loop can react only by adjusting the PWM signal. For example, should the AC line voltage increase, the speed of DC motor M will also increase in response. The motor speed control loop 14 can compensate only by reducing the duty cycle of the PWM signal. Conversely, should the AC line voltage decrease, motor speed control loop 14 can compensate only by increasing the duty cycle of the PWM signal.
The PWM signal is a variable that is calculated and adjusted based upon several parameters, which include the capacitor voltage, the characteristics of the motor, and the operating load point of the motor. The characteristics of the motor and the operating load point of the motor are generally application-defined, and are constants in the control problem. Because the capacitor voltage is unregulated, the turndown of the motor controller is limited by the resolution of the PWM signal, the size of the motor compared to the capacity of the controller, and the line voltage.
The motor drive in FIG. 1 can suffer from limited turndown in certain situations. For example, if motor speed control loop 14 has already defined a high PWM duty cycle for a reason such as low capacitor voltage, and motor M receives a heavy load, then motor speed control loop 14 may not have enough operating range remaining to allow adjustment of the PWM signal to meet this heavy load. Accordingly, there exists a need in the art for a DC chopper motor controller having turndown that is not limited solely by the resolution of the PWM signal, the size of the motor compared to the capacity of the controller, and the line voltage. There exists a further need for a DC chopper motor controller that provides high turndown characteristics by keeping the PWM duty in the approximate middle of its range, thus allowing motor speed control loop 14 full flexibility in moving the PWM duty cycle to either extreme.